1. Field of the Invention
The present invention relates to a laser beam light source which is used for optical recording by means of a light beam in a digital copying machine, a laser beam printer, an optical disc system and the like and, in particular, to a multibeam semiconductor laser array serving as a light source which is applied for so-called multi-beam optical recording.
2. Description of the Related Art
For example, in a laser beam printer, a laser beam from a semiconductor laser is irradiated onto a rotary polyhedral mirror referred to as a polygonal scanner, and the beam reflected from the rotary polyhedral mirror is then irradiated onto the surface of a photoreceptor which moves at a constant speed and is electrostatically charged. Due to the rotation of the rotary polyhedral mirror, the laser beam is scanned in a direction perpendicular to the moving direction of the photoreceptor. Since the laser beam is modulated according to an image to be output, an electrostatic latent image is formed on the photoreceptor. Then, the electrostatic latent image provides a visible toner image when it is developed.
In the laser beam printer of this type, in order to obtain finer resolution of the output image thereof, it is necessary to narrow a distance between scanning lines. Also, in order to output the image at a high speed, the scanning speed must be increased. The greatest problem in obtaining the finer resolution and higher process speed of the laser beam printer is that the rotation speed of polygonal scanner is limited.
In order to solve this problem, a multibeam scanning system in which a plurality of laser beams scan the object surface, is already proposed. It is also a matter of common knowledge that the multibeam scanning system is advantageous not only for application of the laser beam printer but also for improvement of the recording and reproducing speed of an optional disc. In the well known multibeam scanning system, a plurality of laser beam spots must be sufficiently near to one another in a direction (which is hereinafter referred to as a subsidiary scanning direction) perpendicular to a direction to be scanned by the polygonal scanner (which is hereinafter referred to as a main scanning direction). For this purpose, efforts have been made to manufacture a plurality of semiconductor lasers in such a manner that they are near to one another and, for the present, there has been manufactured by way of trial a semiconductor laser array in which semiconductor lasers are made near down to a distance of 10 .mu.m (See Japanese Patent Unexamined Publication No. Hei. 2-39583, R. L. Thornton et. al, "Properties of closely spaced independently addressable lasers fabricated by impurity-induced disordering", Appl. Phys. Lett. 56(17), 1623-1625 (1990) and the like).
On the other hand, in an optical system employed in a laser beam printer using a semiconductor laser whether it uses multibeams or a single beam, it is the most difficult to maintain the accuracy of a collimator disposed on the light source side. The divergence angle of a laser beam light ray emitted from a semiconductor laser is .theta..sub.1 =8.degree. of FWHM (Full Width at Half Maximum) or so in a direction parallel to the junction and .theta..sub.2 =32.degree. or so in a direction perpendicular to the junction (Japanese Patent Unexamined Publication No. Hei. 2-39583). The collimator must be designed in accordance with the large angle, that is, .theta..sub.2, and requires a collimator lens which has a high value of NA (Numerical Aperture) 0.5 or greater. It is difficult to produce a lens of low aberration from such a bright lens. Also, the depth of focus of such a bright lens of NA 0.5 is shallow, that is, on the order of 8 .mu.m, which makes it difficult to maintain the accuracy of the optional system.
The divergence angle of the laser beam of the semiconductor laser depends on the structure of the semiconductor laser and, in view of such dependency, by employing such a structure as having, in the portion thereof in contact with the end surface of a cavity of the semiconductor laser, an area which is formed of a material having a wider band gap than the material of the active layer thereof and also which is not electrically excitable (which structure is hereafter referred to as a window structure), it is reported that the above-mentioned angle .theta..sub.2 reduced down to about one-half, in particular, from 34.degree. down to 19.degree. (See Robert Thornton et. al., "High power (2.1 W) 10 stripe laser arrays with Si disordering facet windows", Appl. Phys. Lett., 49 (23), 1572-1574 (1986)) It is considered that such reduction of .theta..sub.2 is caused by the fact that the laser beam in a window area is spread or diverged to thereby expand a near field pattern on the end surface of the semiconductor laser. If a single beam is used, by means of employment of the window structure, it is then possible to improve the above-mentioned problem that requires a bright collimator lens having NA (Numerical Aperture) of 0.5 or greater, resulting in the shallow depth of focus.
Also, it is well known that the above-mentioned window structure is also advantageous in increasing the output power of the laser beam from the semiconductor laser. The COD (Catastrophic Optical Degradation) level of end surface of semiconductor laser limits maximum power of laser beam. That is, the employment of the window structure can enhance the breaking strength and thus can improve the laser beam output. However, in order to improve the maximum output power by employing the window structure, the window structure must be provided in both of the two cavity end surfaces of the semiconductor laser. This is because, if the COD level of one cavity end surface is enhanced while the COD level of the other cavity end surface remains unchanged, then the breaking strength of the whole semiconductor laser is determined as the lower COD level. In the above-mentioned cured reference as well, it is reported that the maximum output power could be enhanced 4 times higher. It is known that, in an optical disc system, especially in a magnetic optical disc system, the maximum output power required for the laser beam must be large, for example, 100 mW. Therefore, it is considered effective to enhance the maximum output of the semiconductor laser by means of employment of the window structure.
Now, a thermal cross talk provides a problem in the multibeam semiconductor laser array, whether the multibeam semiconductor laser array is used in a laser beam printer or in an optical disc system. In order to improve such thermal cross talk, it is effective to mount the semiconductor laser array tip to a heat sink in a so-called junction-side-down manner. (See, for example, "790 nm High-Power Eight-Beam AlGaAs Semiconductor Laser" written by Bessho et. al. and found in Extended Abstract (Autumn Meeting '91) of The Japan Society of Applied Physics, 11p-ZM-17 (1991), or "Experiments of improving 50-.mu.m-spaced 8 beam laser diode array characteristics by adding a heat-pass-wire" written by Murata et al. and found in Extended Abstract (Autumn Meeting '91) of The Japan Society of Applied Physics, 11p-ZM-18 (1991)). The above term "junction-side-down" means that the junction side of the semiconductor laser is mounted to the heat sink.
To improve the thermal cross talk, it is also believed that increasing the reflectance of the end surface is effective. In particular, if the reflectance of the end surface of the semiconductor laser is increased, then the threshold current of the semiconductor laser is lowered and the characteristic temperature thereof is increased, with the result that the thermal cross talk is reduced. It is also reported that the Droop of a single semiconductor laser element could be reduced down to about 1/5 by increasing the reflectance of the end surface of the semiconductor laser (See "Improvement in Droop Characteristics of Visible Light Laser Diode" written by Shimada et al. and found in Extended Abstract (Autumn Meeting '91) of The Japan Society of Applied Physics, 9p-ZM-3(1991)). The term "Droop" means here a thermal influence on a semiconductor laser element itself when it generates heat. If a semiconductor laser element having a good Droop characteristic is used to provide an array, then it is expected that the thermal cross talk of such array can be reduced. In the above-mentioned paper, it is reported that the Droop could be reduced from 5% down to 1% by increasing by 35% to 50% in the reflectance of both end surfaces of the array. However, in this case, the COD level and slope efficiency may be lowered and, as a result of this, the maximum output power of the semiconductor laser is decreased.
From view of the above description, if the above window structure is employed in the multibeam semiconductor laser array, then the above-mentioned .theta..sub.2 can be reduced and at the same time it is expected that the maximum output power can be increased. It is also considered that the reflectance of the end surface is increased to thereby the thermal cross talk and the decrease in the maximum output power produced as an adverse side effect during this is made up for by means of employment of the window structure.
However, when the above-mentioned window structure is employed in the multibeam semiconductor laser array, then there arises a problem, that is, an optical coupling to be discussed below, which makes it difficult to realize the multibeam semiconductor laser array employing the window structure. The multibeam semiconductor laser array employing the window structure has such a structure as shown in FIG. 15. In such structure, for example, a laser beam 1 emitted from a semiconductor laser element LD-1 is spread or diverged in a window area 2, and is then reflected by an end surface 3 which normally has an optical reflectance of the order of 35%, and part of the leaving ray goes into a semiconductor laser element LD-2 adjacent to the semiconductor laser element LD-1. This means that the two semiconductor laser elements LD-1 and LD-2 are optically coupled together, resulting in an optical cross talk between the two adjoining semiconductor laser elements LD-1 and LD-2. Further, it is considered that, in addition to the increased reflectance of the end surface, the employment of the window structure will worsen the optical coupling problem.
One of methods for decreasing the optical cross talk is to decrease the optical reflectance of the end surface in which the window area is provided. The employment of this method cannot provide an effect to reduce the thermal cross talk but can provide an effect to reduce .theta..sub.2. However, even when only the reduction of .theta..sub.2 is aimed at, if the optical reflectances of both of the end surfaces are reduced, then the quantity of the positive feedback of a laser oscillator is caused to decrease and the threshold current of the laser oscillation of the oscillator is increased and, in the worst case, there is a possibility that the laser oscillator may fail to function as a laser oscillator. As described above, if the window structure is employed so as to reduce .theta..sub.2, then there arises a problem that the optical cross talk between the semiconductor laser elements is increased. Also, if the optical reflectance of the end surface including therein the window area is reduced so as to decrease the optical cross talk, then it arises a problem that the threshold current of the laser oscillation is increased. Further, when there is employed the method of reducing the optical reflectance of the end surface, then it cannot be expected an effect that the maximum output power of the laser beam can be enhanced due to employment of the window structure.